How to use functions from seperate controllers in an MVC framework?

I have learned the fundamental languages for web development, HTML, CSS3, Js, and PHP/ASP. Any who I am on to what I feel like is the next phase, mastering frameworks. Right now I am teaching myself what a MVC framework is, and attempting to learn the basic paradigms that make it such a powerful tool.

I started out learning CodeIgniter 3, thats what is on my server, and in my editor now. So I am bit confused about one thing. First off let me state how I believe mvc and ci3 work.

1. In the url you have your BASEPATH/MY-APP/index.php

then to access a controller you pass it like so ...

2. BASEPATH/MY-APP/index.php/Controller

then you pass parameter to and from the controller to create a dynimic application like so...

3. BASEPATH/MY-APP/index.php/Controller/$1

However, where I get confused is what if I need access to two controllers? Can I extend the controllers to include other controllers?

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